The present invention relates to metering systems and, more particularly, to metering systems having a bypass valve which compensates for fluctuations in fluid supply pressure to maintain a constant pressure drop across a metering valve of the system.
Fluid supply systems such as, for example, fuel supply systems for gas turbine aircraft engines, include a positive displacement pump which generates a relatively high line pressure, a metering valve, and a relatively low pressure supply line extending between the valve and the fuel injectors of the gas turbine engine. With such a system, it is necessary that the pressure differential across the metering valve (metering head) be held constant so that fuel flow is proportional to the controlled metering area of the valve. However, variations in the system during engine operation may cause fluctuations in the pressure of fuel supplied to the metering valve.
Accordingly, such systems include a bypass valve which is responsive to variations in pressure of the fuel in the high pressure supply line, and accordingly "bleeds off" fluid from the high pressure supply line back to a fuel supply upstream of the fuel pump. A typical bypass valve includes a valve body housing a piston which divides a valve chamber into high and low pressure volumes connected to receive fuel from the high and low pressure supply lines, respectively. A reference spring in the low pressure volume urges against the piston to oppose the force exerted on the piston by the high pressure fuel in the high pressure volume.
The bypass valve includes a bypass port communicating with the high pressure volume and connected to a bypass line which in turn is connected upstream of the pump. Should the fuel pressure in the high pressure line increase, the piston in the bypass valve is displaced toward the low pressure volume, progressively uncovering the bypass port which dumps fuel from the high pressure volume and line to reduce the pressure. A disadvantage with such a valve is that the reference spring force varies with the amount of compression of the spring. In order to compensate for this variation, the pressure differential across the piston must decrease as the piston moves to close the bypass port. Such a variation in pressure differential is called "droop." Metering valves with two input motions, such as translation and rotation, generally cannot be modified to compensate for metering head droop.
Attempts have been made to modify the design of the bypass valve to compensate for droop. An example of such a system is disclosed in Wernberg U.S. Pat. No. 4,458,713. That patent discloses a bypass type differential pressure regulator in which an annular pressure chamber is formed between the valve housing and the piston, and is in fluid communication with a chamber within the piston that communicates with the high pressure volume through a restrictive orifice. As the piston is displaced toward the low pressure volume to uncover the high pressure bypass port progressively, the pressure drop resulting from flow through the orifice into the piston chamber reduces the pressure transmitted to the chamber and thereby reduces the total force acting to displace the piston toward the high pressure volume. As bypass flow increases, this pressure becomes progressively less so as to offset the progressively increasing spring force.
A disadvantage with such systems is that a number of orifices required creates a higher likelihood of clogging and performance out of a desired range. Further, such devices require highly complex pistons and seals which add significantly to the overall cost. Accordingly, there is a need for a bypass-type differential pressure regulator which is relatively simple in construction and yet provides reliable performance.